Re: [PATCH v2 for-5.0 0/8] ppc: Consolidate QOM links and pointers to the same object

[Greg Kurz <groug@kaod.org>]

On Mon, 18 Nov 2019 10:26:12 +0100
Markus Armbruster <armbru@redhat.com> wrote:

> Greg Kurz <groug@kaod.org> writes:
> 
> > There's a recurring pattern in the code where a const link is added to a
> > newly instanciated object and the link is then used in the object's realize
> > function to keep a pointer to the QOM entity which the link points to.
> >
> > void create_obj_b(Object *obj_a)
> > {
> >     Object *obj_b;
> >
> >     obj_b = object_new(TYPE_B);
> >     object_property_add_const_link(obj_b, "link-to-a", obj_a, &error_abort);
> >     object_property_set_bool(obj_b, true, "realized", &error_abort);
> > }
> >
> > void object_b_realize(DeviceState *dev, Error **errp)
> > {
> >     Object *obj_a;
> >
> >     obj_a = object_property_get_link(OBJECT(dev), "link-to-a", errp);
> >     if (!obj_a) {
> >         return;
> >     }
> >
> >     obj_b->obj_a = A(obj_a); // If obj_b->obj_a is changed, the link property
> >                              // still points to the original obj_a that was
> >                              // passed to object_property_add_const_link()
> > }
> >
> > Confusing bugs could arise if the pointer and the link go out of sync for
> > some reason. This can be avoided if the property is defined to directly use
> > the pointer with the DEFINE_PROP_LINK() macro.
> >
> > This series just does that for all occurences of the fragile pattern in
> > the XIVE and PNV code.
> 
> Have you looked for the pattern elsewhere?
> 

No I was focusing on the XICS/XIVE interrupt controller code for PPC machines
only. I'll have a broader look.